Origin of porosity in evaporite platform dolostone in the middle Cambrian Shaelek Formation, NW Tarim Basin, China: constraints from petrology, mineralogy and geochemistry

The extensive middle Cambrian evaporite platform dolostone represents a significant area for hydrocarbon exploration in the Tarim Basin with recent advances indicating promising exploration prospects. However, owing to its considerable depth and the limited exploration, the genesis and distribution of evaporite-related dolostone reservoirs from the middle Cambrian remain poorly understood. This study includes detailed field investigations along with petrological, mineralogical and geochemical analyses to characterise the secondary porosity of the middle Cambrian Shaelek Formation evaporite platform dolostone and to explore the origin of the diagenetic fluids responsible for these features. The results show that gypsum dissolution vugs are the main secondary porosity in the evaporite platform dolostone reservoirs, with their development and distribution strongly controlled by sedimentary facies. Meteoric dissolution during the penecontemporaneous period is responsible for this secondary porosity. The frequent fluctuation in sea-levels and the transient exposure of the dolostone reservoirs provide favourable conditions for the dissolution of unstable gypsum. The multiple stages of calcite filling in the vugs result from supersaturated precipitation from fluids during the late stage of meteoric diagenesis. Variations in delta C-13, delta O-18 and rare earth element compositions between the two types of calcite reflect different stages of fluid evolution and varying degrees of water-rock reaction. The vug-filling fluorite is of non-hydrothermal genesis but results from the highly evaporative concentrated seawater and terrigenous input. The highly concentrated F- in evaporative seawater, combined with F- from fluvial input (if any) during subaerial exposure, most likely provides the F- necessary for fluorite precipitation. A model of reservoir formation and evolution has been established that will be beneficial for hydrocarbon exploration and prediction of the deeply buried Cambrian evaporite platform dolostone reservoir.